The present invention relates to rapid testing for elemental materials and in particular relates to rapid testing for Group I and Group II elements in food products.
The composition of food products is a topic of significant interest on several levels, particularly when the food is prepared or prepackaged; e.g., canned, frozen, refrigerated, or otherwise provided to the consumer in a modern food supply chain environment. In addition to consumer preferences such as taste, appearance, and price; legal regulations in the United States (and similar regulations elsewhere in the world) require the packaged food be labeled with accurate information about its contents.
Frequent candidates for compositional analysis include fats and oils, proteins, carbohydrates, synthetic additives, and trace elements. Sodium, typically present as sodium chloride (NaCl; “salt”) is of particular interest. In addition to its usual recognition as enhancing the taste of food, sodium is a necessary element for sustaining life and good health, and is a well-known preservative for many foods. Sodium is also present, however, as sodium bicarbonate in other common food agents such as baking powder and baking soda
The skilled person recognizes that although the word “salt” is widely used to refer to sodium chloride, the term has a more precise meaning in a formal chemistry sense and covers any compound that is generally formed of alternating positive and negative ions, usually in an ordered crystal structure.
Nevertheless, an excess of dietary sodium has been linked (directly or indirectly) to undesired health consequences such as hypertension (“high blood pressure”), heart attacks, stroke, heart failure, and other cardiovascular disease. Other potential (although more tenuous) problems may include higher risks for osteoporosis or cancer.
In some prepared foods, sodium is not naturally present, and thus can be added carefully in desired measured amounts. In other foods, however, the amount of sodium can vary widely (relatively speaking) and be less predictable to the packager or the consumer on a case-by-case or item-by-item basis. As examples, higher amounts of salt are often present in cheese, processed meats, snack foods, and canned soup.
Alternatively, because salt is a preservative, reducing the total amount of sodium in a given food product will change—and typically shorten—the shelf life of that product. The potassium ion (K+1) can replace the sodium ion (Na+1) for some purposes, but has different taste qualities than sodium and thus cannot simply replace sodium in the absence of some other adjustment or result.
Nevertheless, because potassium chloride (KCl) is replacing some or all of the sodium chloride in a number of food products, the presence and amount of potassium represents valuable information.
Calcium content is also important in a number of food products and is particularly important in dairy products because it affects several relevant properties, and particularly affects the properties of cheese. Calcium is important in the rendering aspect of the poultry industry, because the measurement of calcium is generally used as the most accurate method of determining the bone content in (for example) animal feed meal made from poultry products.
Lithium content is important in both medical and industrial applications. Barium has no known health benefits (the human body contains about 0.00003%) and instead represents a health risk if present in more than trace amounts
The effort to raise, lower, or maintain an appropriate level of an element such as sodium in a food product thus requires testing. Conventional sodium testing, however, presents disadvantages, and in some cases problems, in the food preparation and packaging context.
For example, the Food Safety and Inspection Service (FSIS) of the U.S. Department of agriculture (USDA) recently approved a sodium test that includes at least about 4 hours of ramp time and 15 hours of dwell time (CLG-SOCAL3.00; “Determination of Sodium and Calcium by ICP Atomic Emission Spectroscopy;” Sep. 25, 2014).
In the preparation of large quantities of packaged or prepared foods, longer testing times raise several disadvantages or problems. First, fewer tests can be carried out over any given time interval, thus reducing the amount of data produced and available. Second if the packaging or production steps are carried out on a continuous or near-continuous basis, the time interval needed for testing requires that (1) production be halted until an answer is determined, or (2) if production continues while testing is carried out, the testing interval creates a risk that a large proportion of undesired product will be made during the testing interval
Sodium is traditionally tested (quantitatively measured) by titrating with a silver nitrate (AgNO3) solution of known concentration that will react with the chloride ion (Cl−1) in sodium chloride to produce a solid precipitate, and potentially with a potassium chromate solution (K2CrO4) to add a color change at the endpoint of the silver chloride reaction.
Although titration is rapid, it is actually a test for chloride ion (Cl−1) content based upon the assumption that the chloride ion is entirely present as sodium chloride (NaCl). Thus, if all of the chloride is indeed present as sodium chloride the test is accurate, but when another salt is the chloride ion source, the assumption breaks down and the test results for sodium are inaccurate. Stated differently, the titration test cannot distinguish between or among sodium chloride, potassium chloride, or lithium chloride.
Sodium in food can also be determined by now-conventional microwave digestion in acid. Although microwave digestion techniques offer advantages (for example the use of acids at temperatures above their boiling point in sealed vessels), and although microwave heating is generally much faster than conventional conductive or convection heating, the steps required for sample preparation, digestion, filtration, dilution and analysis, nevertheless can take up to an hour for each sample. In particular, acid digestion is a relatively slow process even with microwave energy because the steps of ramping the temperature, holding the temperature, and cooling thereafter, are required regardless of the speed of the heating step. The concentrated acids used in acid digestion are also disadvantageous, or inappropriate, or simply too complex to be conveniently handled in a normal plant environment.
Sodium content testing can also be carried out by dry ashing using microwave techniques, but in some cases the ashing step must be carried out for as long as 12-24 hours; i.e., an interval far too inconvenient for rapid food testing purposes.
Carrying out digestion techniques at even higher temperatures can increase speeds, but can also reduce the amount of recovered element in a form that can be tested, thus sacrificing accuracy. Furthermore, the longer a sample is heated, and the higher the temperature to which it is heated, the longer the time required before the sample can be handled for solution and spectroscopy purposes.
A number of elements begin to volatilize as temperatures approach or exceed 500° C. and in a testing protocol that seeks to capture an element, such a volatile loss produces inaccurate results.
Faster (more rapid) tests are available, but also present specific disadvantages. A recent option is X-ray fluorescence. As a brief explanation of the technique, high energy x-rays or gamma rays are used to displace electrons from inner orbitals of elemental atoms in a sample. When another electron drops into the empty orbital from a higher energy orbital, the transition generates a fluorescent photon with a characteristic energy. The detection and analysis of these fluorescent photons provide a quantitative measurement of the amount of the element in the original sample.
Although accurate and powerful, X-ray fluorescence presents disadvantages in the food testing context. The method is sensitive to the matrix in which the element is found, generates heat, requires sophisticated sample preparation, generally requires a state or Federal license, and uses large and expensive instruments. In particular, the X-ray fluorescence test must be adjusted or customized for almost every different food product being tested; i.e., hundreds of protocols are required.
Therefore, a need remains for a less expensive, less complex, and (perhaps most importantly) more rapid technique for accurately determining the proportional quantity of elements—particularly sodium—in compositions, and particularly food compositions.